FIGS. 1A and 1B are cross sectional views showing processes of a conventional method of forming trench buried wiring on a semiconductor device. As shown in FIG. 1A, a thermal oxidization film 2 is formed on a semiconductor substrate 1. Formed thereafter is a trench (an opening in the thermal oxidation film) 3 having a rectangular cross section. Thereafter, electrode material 4 such as Al alloy or W is deposited to a sufficient thickness. Then, the whole surface is etched back, or it is etched back after a flattening coating film 6 such as a resist film, is coated thereon. As a result, as shown in FIG. 1B, the electrode material 4 on the surface of the thermal oxidation film 2 is removed, leaving the electrode material (metal) 4 forming an electrode 5 only within the thermal oxidation film opening 3.
With the above-described conventional method, the electrode material 4 used is unnecessarily thick, while almost all the electrode material 4 at the area other than the thermal oxidization film opening 3 is etched back. Namely, the material efficiency is very poor.
Tungsten or the like used as the metal material (electrode material 4) provides relatively easy dry etching. However, using Cu as the metal material is very difficult in forming Cu wiring through an RIE method using C1-based gas. No ion milling technique is known which does not use reaction gas. However, this technique may damage semiconductor substrates.
With the etching-back method, it is possible to leave the electrode material 5 within the thermal oxidization opening 3 if it is relatively small. However, as shown in the cross section of FIG. 2, if a thermal oxidization film opening 3a is used which is larger than the thermal oxidization film opening 3, the electrode material 5 is etched greatly, leaving the electrode material 4 as side walls 5a. It is therefore difficult to form wiring by leaving electrode material or the like within the large opening 3a.